Soft tissue mechanics of the temporomandibular joint

Direct measurement of temporomandibular joint (TMJ) tissue deformation requires animal experimentation. Most of the available data pertain to the mechanical strain on the bone surfaces around the joint. However, bone is rarely the first joint tissue to show injury, being affected after damage to collagenous tissues such as the disc or capsule. Capsular ligaments guide or limit movement, while the intra-articular disc may also distribute joint loads. However, these tissues are difficult to visualize dynamically and not suitable for strain gage attachment, so in vivo deformations are poorly understood. Using pigs as the best nonprimate model for human TMJ function, we implanted differential variable reluctance transducers to measure antero-posterior strain in the lateral aspect of the intra-articular disc. The results were compared to previously published data on the TMJ capsule. Passive manipulation in anesthetized animals indicated that opening, protrusion, and contralateral movements caused the disc to elongate. On the contrary, closing, retrusion and ipsilateral movements caused disc shortening. These strains are opposite to those observed in the capsule and are expected on anatomical grounds. Surprisingly, disc strain during mastication differed from that during manipulation. The disc elongated during jaw closure, more on the retruding balancing side (16% +/- 1) than on the working side (8% +/- 2). This anomalous behavior may reflect compressive loading, such that the disc elongates as a result of the Poisson effect rather than condylar movement. Because the capsule also elongates during the power stroke, especially on the balancing side, both disc and capsule are maximally loaded on the same side at the same moment.